Process for the drying of timber or other materials



July 31, 1956 c. E. DIXON 2,756,513

PROCESS FOR THE DRYING OF TIMBER oR OTHER MATERIALS Filed Aug. 27. 1952 9710/7 /&

INVeMTO/a 6 6081.55 5001M Q/AOA 1 I W, 44A/ z United States Patent PROCESS FOR THE DRYING F TIMBER OR OTHER MATERIALS Charles Edwin Dixon, Armadale, Victoria, Australia Application August 27, 1952, Serial No. 306,668

Claims. (Cl. 3437) This invention relates to an improved method for drying and reconditioning timber.

One known process for the drying of timber is carried out in a ventilated kiln at relatively low temperatures. Air is circulated through the kiln and heated by various means such as steam pipes. This process is slow in operation and is difficult to operate efliciently.

In more recent modifications of this process, the drying is carried out in a ventilated kiln, but at higher temperatures (up to 240 F.).

In the vapour drying or solvent seasoning process, air is excluded from the kiln as far as possible and solvent (working fluid) is circulated in vapour form at high temperatures. Although this process gives a faster drying rate than the low temperature ventilated kiln type of drying, there are several disadvantages inherent in its use. These are (a) explosion and fire risk due to the nature of the volatile solvent; (b) the solvent being relatively expensive, the operational costs are high and (c) little (if any) flexibility of control as the process is normally carried out at the boiling point of the solvent.

One object of the present invention is to provide a process for the drying of timber which can be carried out more rapidly (and consequently more economically) and with greater simplicity of control than the first-mentioned known process.

A further object is to provide a process which substantially eliminates the fire and explosion risk inherent in the solvent seasoning process and which allows a greater degree of flexibility in the operating conditions than is normally possible in the latter process.

A still further object is to provide a process whereby heat from the products of the process can be effectively utilised.

Another object is to provide a process enabling useful products to be obtained from the drying process much more readily than has heretofore been possible.

Further objects will become apparent from the ensu I ing description.

According to my invention the materialto be dried is dried in and by a completely air-free atmosphere of superheated steam at a temperature of from 240 F. to 400 F and produced wholly or mainly from themoisture givon off from the material being dried. However, live steam produced externally may be sprayed into the drying chamber, or alternatively water may be sprayed on to the heating pipes in the kiln, particularly for the treatment or reconditioning of refractory hardwoods and the removal of collapse. drying rate is effected by varying the quality of the steam whereas in known processes in which air is not excluded, control is effected by varying the relative humidity.

Normally the kiln will be operated at atmospheric pres sure but the process of the invention can be carried out.

at other pressures if expedient.

The process of my invention is carried out in a non-ventilated air-tight kiln heated by means of flue gases which In this process control of the 2,756,513 Ce Patented July 31, 1956 are passed through fire tubes in the kiln chamber. The gaseous products of the process (efliuent) are led oil through one or more ducts and may be passed firstly through a heat exchanger to heat broiler-feed water, water to be sprayed on to the fire tubes in the kiln, or for other heating purposes, secondly through a condenser wherein further heat may be extracted and thirdly through a gravity separator in which useful products such as pine oils readily separate from the condensed aqueous medium and may be collected;

To facilitate the detailed description of the preferred form of kiln in which the process of the invention may be carried into practical effect, reference will now be made to the accompanying drawings in which:

Figure 2 is a sectional plan view of the kiln shown in V Figure l.

The kiln is made preferably of welded sheet metal or galvanised iron stiifened with angle iron or other structural members and is insulated with mineral wool and an outer layer of wood shavings or the like. The inner surfaces of the kiln chamber are coated with an inert preservative material such as phenol-formaldehyde resin or lacquer. The kiln is constructed so as to be an airtight, non-ventilated structure having a safety U-trap and drain (not shown) in the base thereof. The drying chamber 1 in which the wood is stacked for drying is heated by a series of heatingcoils 3 connected with the flue outlet 8 from a step grate'7by means of flue d. The drying chamber 1 is provided with'a loading door 2 which opens on to a trench 5 below the level of ground 6. The flue gases after passing through the coils 3, enter a further series of heating coils" 13 through header 4 and then pass to the stack 12 through flue 10. The heating coils 3, 13 are arranged to give uniformity of heat transfer throughout the chamber'and are disposed below a false floor in the form of a bafiie 21' which extends the full length of the drying chamber 1 but is clear from each sidewall thereof. A by-pass flue 11 directly connects the outlet line 8 with the stack 12, the path of the flue gases being controlled by means of a baffle 19 which is mounted for-pivotal movements in the flue 10 at the junction of the latter and the by-pass flue 11.

An outlet duct 14 leads oil from the upper part of the drying chamber 1, passes firstly through a heat exchanger 15 and then leads into a ,U -type gravity separator having a collecting tank 16 and outlets 17, 18. A pipe 20 from which steam may be sprayed into the drying chamber 1 or from which water may be sprayed onto the heating pipes is also provided.

In accordance with the invention, the drying is effected in and by a completely air-free atmosphere of steam which is derived wholly or mainly from the moisture given off by the material being dried and is superheated above the saturation temperature to a degree necessary to give the required drying rate. The range of drying temperatures of the kiln atmosphere varies between the 240 F. and 400 F. and is preferably between 300 F. and 400 F. No air is present after the initial air has been expelledfrom the kilnand from this stage until the temperature rises above 212 F (at atmospheric pressure) no efiective drying takes place. Steam of low quality or at saturation may result in the reverse of drying, i. e. the raisingof the moisture content of the timber being dried, particularly if the timber is at low moisture content. By raising the quality of the steam, drying of the timber may be etlected. I v

The temperature of the drying chamber is controlled by operation of .bafiie 19. If thelatter is pivoted to a position whereby the by-pass flue 11 is closed, all the flue mnesia.

gases will pass from the grate 7 through the heating coils 3, 13 and the full heating elfect of the flue gases will'be felt in the drying chamber. If, however, the baflle 19 ismoved so as cL se-th lue l0, al h time w l pas i iir c y f m th grate 7 to th ta k-l2 and'the flue g e n ha :tht ugh :the hea n reqi al In int rmedi t pos ti n th hafli g-a ,netti n {at the flu e e ill pa s .h gh he a i g fihr ni e an it is e vi ha eon ql hot th e nn atute {d yulb in the hea ing chamber ean' be .0i tr0. ed 'hy ui a le manipulation of the bafije.

By u l sing high t mp rame +up t 400 =advantage can be taken of rapid transtusion of moisture mush t e woq ;of th at-eate pla ticity ot e atsn temp t tut e- B cent e 'ot he d y bu b: mne atu egi ny dryin ra item zero to the maximum possible for the material ,can be obtained. 7

Thus the ,drybulb temperature isthe-controlling factor and the process can be readily controlled to produceoptimum d y n o iti ns vas requ r d t r th particul r material being treated. This method of control stands in obvious and tavourable contrast totearlier :methods wherein the relative humiditypf the atmospherein the kiln controlled thedrying rate ofitltle charge. of that type, complicated tables vof relative wet and dry bulb temperatures were required for cpntrolpurposes and a relatively small vchange in, the wet lbulb depression often caused a relatively large ehange inthe drying conditions in the kiln. Th s, in theold processes, twp temperatures (or one temperateureanda wet bulbdepression) were necessary to describe the schedule or drying conditions and these temperatures were not related .to a base temperature. In the p esent process, however, there ,is a base temperature, namely the saturation temperature s y app x) and the pa ticu a temp atur (dry bulb) containing at any time therefiore completely describes the schedule or drying conditions at thattirne.

l n eat o th u h i m th ry g eh mh rrmay e used onv e y t r t -hea boi r e'eld wa e r-e f yi amb r r t ,measut men lot t enu aflo h i en ion hu ren me d y n an reesmtli ie ia of timber to be carried out more economically ,an in n i dkiln y-th sql ent e sqni g-pmeess, Th higher drying temperatu s. not the. presei pme s 'su ly edu th dry ng m sieemnar d w h r a me n ent a d h ns-wh le-theteie no l sst f e pen iy e t a v u n t e anqur tse sohi g ro esa h xpl io a fin k Q s im- "YP Q 'Q Sv isa s e processes i e y v am vby my ni thedth s ,ahvious that thep se n nti 'zhaamanya a tage t ner 'm hods .at present in use.

I claim:

1. A method of ,ran s y y n g and r phtl nni ber, such drying and reconditioning being-.cazngied out-atone n h sam e, hyte andIt esame nro e andin ne d t e am v.el a be by heat ih ssai in the complete absenc 9' ai .by me -atrium ,flue v the kiln charge while still present inth'e said'kiln chamber and until all the air originally present in the said kiln chamber is expelled by the expansion caused by the evaporation of the water from the kiln charge, no further drying then taking place until the temperature in the said kiln chamber has been raised above the boiling point of water at the location of the kiln, at which time the kiln atmosphere, being completely free of air, st whqlly .Q wa er vapqr 9r uperhea ed st am a wood extractives, the saidlwater vapour or superheated steam and wood extractives being obtained solely from the .-.-l-;i;ln e a ge s.elthe system sheins a enmple ely ,r circulatory closed one the kiln chamber, in which drying and reconditioning take place concurrently, the reconditioning and the rate of --,drying being controlled by the degree of superheat of the said steam or water vapour as indicated by the dry-bulb temperature alone, the wet-bulb temperature remaining constant at the boiling p in o h e th .saidd y-hull: tempe ature h i s controlled by controlling the llow oi the piped flue gases pa s ng th h th k ln-cham r so tha th ai dryin a e i ht ai a d the de re sch dul i m inta e ire ly n o e y by entrol ihg heil w at the aid Piped u a e d the det nm n ieu of h :PEOEEQS o the drying Process b h btaineda d eliv d t a th mea urem h f he t f ent t amtthez aid kil ,e amhe n fu h c m ris n xhahstinsthe eam as gi en eff.

2., A method of rapidly drying and reconditioning tin er, uehid y hs a d r eand ti ni s e ng earr e 'e a n an t esam timeby .one. and the same pra ti e-an in Q a t e ame ln chambe y a g th sai ln s am-h r t temperature :t en12 0 to 00 F. in t 1c mp et abs ce-Q a y me n o p ped ue ga s pass :th eugh hesaid ki n ch mb r n sh l ma e as g ve nn Q mi-ty q ihea tr sfer thre ghn th a d ln ham h h a thu ,P edueed in he said i n. ham er fi flyaus ng W ter-to e yaperat d ram. h .k ln charge whi e ai s il p esent n th eid vor super-heated steam introduced gins thkiln chamber,

h isystem heing a complet ly r -circu a o y cl sed. o e thin. the sa d ha i h whi h dryi g and .5 01 ditioning take place, concurrently, the reconditioning bens een re led n rtially by h tqual ty o t e wa er inn rTsun -h a .d steamihtrodmed we h said ki -sham and the ra e Q d yingheing ep rqll a he ee ntinnine ipaItia ly .enntrell d y th e re of s pe -h a of the kiln atmosphere as indicated by the dryfblulb tem- Pe athr a pne, thewee ii b tempe a u e remainin -en ahta theh t emp ature b g .e nt ql ed by eQntm n th .fl wlefr h -p nedflue. a e

haty he. sa dd y arat is leti tlled, er eeiii i iei i gw Partia ly at lledwands t e desir d ehedule mainta n d ir et y and se ely y nn rel h th flow f th said,

p pedflh ga es and-th e ermin ti n 0f th pro re s f t e d y ng pmee s .heingpbtain r and de i e mrn without the use of solvents for thesaidextragtiyes;

4. A eth d a ran lyfls and re ens itien n m er such d ying nd ee adit rqni eingrear ied .Qut at one and the same time,by one and the same process ,g ,p i pt the wat r, t e sa d d yul and in one and the same kiln chamber, by heating the said kiln chamber to temperatures from 240 F. to 400 F. in the complete absence of air by means of piped flue gases passed through the said kiln chamber in such manner as to give uniformity of heat transfer throughout the said kiln chamber, the heat thus produced in the said kiln chamber firstly causing water to be evaporated from the kiln charge while air is still present in the said kiln chamber and until all the air originally present in the said kiln chamber is expelled by the expansion caused by the evaporation of the Water from the kiln charge, no further drying then taking place until the temperature in the said kiln chamber has been raised above the boiling point of water at the location of the kiln, at which time the kiln atmosphere, being completely free of air, consists Wholly of Water vapour or super-heated steam and wood extractives, the said water vapour or superheated steam and wood extractives being obtained solely from the kiln charge itself, the system being a completely re-circulatory closed one within the kiln chamber,

in which drying and reconditioning take place concurrently, the reconditioning and the rate of drying being controlled by the degree of super-heat of the said steam or water vapour as indicated by the dry-bulb temperature alone, the wet-bulb temperature remaining constant at the boiling point of the water, the said dry-bulb temperature being controlled by controlling the flow of the piped flue gases passing through the kiln chamber so that the said drying rate is controlled and the desired schedule is maintained directly and solely by controlling the flow of the said piped flue gases and the determination of the progress of the drying process being obtained and derived from the measurement of the eflluent from the said kiln chamber, the vapourised wood extractives being condensed directly from the kiln atmosphere and collected and further comprising exhausting the steam as given 0E.

5. A method of rapidly drying and reconditioning timber, such drying and reconditioning being carried out at one and the same time, by one and the same process and in one and the same kiln chamber, by heating the said kiln chamber to temperatures from 240 F. to 400 F. in the complete absence of air by means of piped flue gases passed through the said kiln chamber in such manner as to give uniformity of heat transfer throughout the said kiln chamber, the heat thus produced in the said kiln chamber firstly causing water to be evaporated from the kiln charge while air is still present in the said kiln chamber and until all the air originally present in the said kiln chamber is expelled by the expansion caused by the evaporation of the Water from the kiln charge, no further drying then taking place until the temperature in the said kiln chamber has been raised above the boiling point of water at the location of the kiln, at which time the kiln atmosphere, being completely free of air, consists mainly of water vapour or super-heated steam and wood extractives obtained from the kiln charge itself, the remainder of the kiln atmosphere comprising water vapour or super-heated steam introduced into the kiln chamber, the system being a completely re-circulatory closed one Within the said kiln chamber, in which drying and reconditioning take place concurrently, the reconditioning being controlled partially by the quality of the Water vapour or super-heated steam introduced into the said kiln chamber, and the rate of drying being controlled and the reconditioning partially controlled by the degree of superheat of the kiln atmosphere as indicated by the dry-bulb temperature alone, the wet-bulb temperature remaining constant at the boiling point of the water, the said dry-bulb temperature being controlled by controlling the flow of the piped flue gases passing through the kiln chamber so that the said drying rate is controlled, the reconditioning partially controlled and the desired schedule maintained directly and solely by controlling the flow of the said piped flue gases and the determination of the progress of the drying process being obtained and derived from the measurement of the out-flow from, and the inflow into, the said kiln chamber, the vapourised wood extractives being condensed directly from the kiln atmosphere and collected and further comprising exhausting the steam as given ofi.

References Cited in the file of this patent UNITED STATES PATENTS 649,758 Robertson May 15, 1900 1,059,820 Besemfelder Apr. 22, 1913 1,678,559 Devine July 24, 1928 2,119,261 Andrews May 31, 1938 2,127,638 Brandwood Aug. 23, 1938 2,376,612 Muench et al. May 22, 1945 2,446,147 Walund July 27, 1948 2,548,403 Smith Apr. 10, 1951 2,618,560 Leslie Nov. 18, 1952 OTHER REFERENCES Special Methods of Seasoning Wood, High-Temperature Drying. Forest Products Laboratory Report 1665-1. Revised May 1954, 5 pages.

Special Methods of Seasoning Wood, Superheated Steam. Forest Products Laboratory Report #R1665-1. March 1947, 1 page. 

